Heat exchanger

ABSTRACT

A condensing heat exchanger of the kind having a primary heat exchanger and a secondary heat exchanger connected downstream of and in series with the primary heat exchanger. The secondary heat exchanger is arranged to at least partially condense combustion gases discharged from the primary heat exchanger. The primary heat exchanger comprising a drum having a longitudinal axis and the secondary heat exchanger comprising a plurality of small diameter tubes arranged by the side of the drum and extending parallel with the longitudinal axis of the drum.

This disclosure relates to a heat exchanger, more particularly a condensing type heat exchanger. The disclosure also relates to a method of assembling a heat exchanger, more particularly a condensing type heat exchanger.

A known fuel-fired forced air heat exchanger is set forth in U.S. Pat. No. 4,960,102. The heat exchanger includes a primary heat exchanger and a secondary heat exchanger connected downstream of and in series with the primary heat exchanger. Advantageously, the secondary heat exchanger is arranged to condense the combustion gases discharged from the primary heat exchanger, and so capture latent heat from the available combustion gas. Such heat exchangers are commonly referred to as “condensing” heat exchangers.

The increasing cost of fuel and materials means that there is a need to improve the heating efficiency and assembly of such heat exchangers.

According to one aspect of the invention, there is provided a condensing heat exchanger of the kind having a primary heat exchanger and a secondary heat exchanger connected downstream of and in series with the primary heat exchanger, in which the secondary heat exchanger is arranged to at least partially condense combustion gases discharged from the primary heat exchanger, wherein the primary heat exchanger comprises a drum having a longitudinal axis and the secondary heat exchanger comprises a plurality of tubes extending parallel with the longitudinal axis of the drum.

In exemplary embodiments, the tubes are provided in an array adjacent, e.g. to the side of, the drum.

In exemplary embodiments, a spiral formation is formed in the outer surface of each tube, for promoting flow of combustion gases through the tube.

In exemplary embodiments, the spiral formation begins at a predetermined distance from one end of the tube and stops at a predetermined distance from the opposite end of the tube, so that the ends of the tube are plain.

In exemplary embodiments, the tubes have a small diameter relative to the diameter of the drum.

In exemplary embodiments, the tubes have a diameter in the range 4 mm to 8 mm. In another embodiment the tubes have a diameter in the range 5 mm to 7 mm. In a further embodiment the tubes have a diameter of 6 mm.

In exemplary embodiments, the tubes are arranged in staggered rows.

In exemplary embodiments, the tubes are provided between inlet and outlet plates, as a subassembly.

In exemplary embodiments, the inlet and/or outlet plates are preformed with tube apertures for receiving the ends of the tubes in a pre-defined array.

In exemplary embodiments, the tube apertures have a peripheral flange projecting from the associated plate to provide a sleeve for a section of the tubes.

In exemplary embodiments, the primary heat exchanger consists of a subassembly including a cylindrical drum sealingly attached to an outlet plate; wherein the secondary heat exchanger consists of a subassembly including said plurality of tubes sealing attached to an inlet plate; and wherein the two subassemblies are united with one another by attachment between the outlet plate and inlet plate.

In exemplary embodiments, a closure is attached to the united subassemblies and defines a passageway for combustion gases from the primary heat exchanger to the secondary heat exchanger.

In exemplary embodiments, an array of concentric circles pressed into a surface of the closure to reduce noise during flexure of the closure under thermal expansion.

In exemplary embodiments, the closure is of box-type configuration having four side walls and a base wall, wherein the base wall is pressed outwards, to extend beyond the side walls, and defines four generally triangular sloping surfaces.

According to another aspect of the invention, there is provided a method of assembling a condensing heat exchanger of the kind having a primary heat exchanger and a secondary heat exchanger connected downstream of and in series with the primary heat exchanger, and in which the secondary heat exchanger is arranged to at least partially condense combustion gases discharged from the primary heat exchanger, comprising the steps of:

-   -   providing the primary heat exchanger in the form of a drum         having a longitudinal axis, and providing the secondary heat         exchanger in the form of a plurality of tubes extending parallel         with the longitudinal axis of the drum.

In exemplary embodiments, the method includes the step of providing an inlet plate and an outlet plate for the tubes, the inlet and outlet plates being pre-formed with tube apertures for receiving the ends of the tubes in a pre-defined array, arranging the plates in a spaced array with said tubes arranged between the plates with the tubes in alignment with the tube apertures, and driving the plates together in the direction of one another to force the ends of the tubes into the respective tube apertures.

In exemplary embodiments, each tube aperture is provided with a peripheral flange projecting from the associated plate to provide a sleeve for a section of a respective tube.

In exemplary embodiments, the peripheral flange is swaged to fixedly couple the flange to the tubes.

In exemplary embodiments, a spiral thread is formed in the outer surface of each tube prior to incorporation in the assembly.

In exemplary embodiments, each tube is drawn and then the spiral thread is formed on the drawn tube, prior to a second drawing operation, to remove any significant deformations generated when the thread is formed.

In exemplary embodiments, the primary heat exchanger is formed as a subassembly including a cylindrical drum sealingly attached to an outlet plate; the secondary heat exchanger is formed as a separate subassembly including said plurality of tubes sealing attached to an inlet plate; and the two subassemblies are united with one another by attachment between the outlet plate and inlet plate.

In exemplary embodiments, a closure is attached to the united subassemblies to define a passageway for combustion gases from the primary heat exchanger to the secondary heat exchanger.

In exemplary embodiments, an array of concentric circles pressed into a surface of the closure to reduce noise during flexure of the closure under thermal expansion.

In exemplary embodiments, the closure is of pre-formed box-type configuration having four side walls and a base wall, wherein the base wall is pressed outwards, to extend beyond the side walls, and defines four generally triangular sloping surfaces.

In a further aspect of the invention, there is provided a condensing heat exchanger of the kind having a primary heat exchanger and a secondary heat exchanger connected downstream of and in series with the primary heat exchanger, in which the secondary heat exchanger is arranged to at least partially condense combustion gases discharged from the primary heat exchanger, wherein the primary heat exchanger comprises a drum having a longitudinal axis and the secondary heat exchanger comprises a plurality of tubes extending parallel with the longitudinal axis of the drum, wherein a spiral formation is formed in the outer surface of each tube, for promoting flow of combustion gases through the tube.

Other aspects and features of the invention will be apparent from the attached claims and the following description of preferred embodiments, made by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic perspective view of an outlet plate for a secondary heat exchanger;

FIG. 2 is an enlarged view of encircled region A from FIG. 1;

FIG. 3 is a schematic perspective view of an inlet plate for a secondary heat exchanger;

FIG. 4 is an enlarged view of encircled region B from FIG. 3;

FIG. 5 is a schematic perspective view of a secondary heat exchanger subassembly incorporating the inlet and outlet plates of FIGS. 1 and 3;

FIG. 6A is schematic side view of a heat exchanger tube for use in the subassembly of FIG. 5;

FIG. 6B is an enlarged view of encircled region C from FIG. 6A;

FIG. 6C is schematic cross-section through the heat exchanger tube of FIG. 6A;

FIG. 7 is a schematic perspective view of a primary heat exchanger subassembly;

FIG. 8 is a schematic perspective view showing the pre-assembly formed when the subassembly of FIG. 5 is united with the subassembly of FIG. 7;

FIG. 9 is a schematic perspective view of the pre-assembly of FIG. 8 prior to attachment of a cover element to provide a passageway between the primary heat exchanger and the secondary heat exchanger;

FIG. 10A is schematic side view of the closure from FIG. 9; and

FIG. 10B is cross-sectional view of the closure from FIG. 10A taken along line D-D.

Referring firstly to FIG. 1, there is shown a plate 10 of rectangular form. The plate includes an array of primary fixing apertures 12 arranged about the periphery of the plate 10, e.g. one at each corner 14 of the plate 10 and one positioned mid way along each of the two longest sides 16 of the plate 10. The plate 10 also includes a plurality of secondary fixing apertures 18 arranged in two rows, each row being spaced inwardly of the primary fixing apertures 12 and extending in a direction parallel with the respective longest sides 16 of the plate 10.

The plate 10 further includes an array of tube apertures 20 arranged centrally on the plate 10 in a plurality of rows extending in a direction parallel with line the longest sides 16 of the plate 10. Adjacent rows in the array of tube apertures 20 are staggered relative to one another, e.g. so that the apertures do not align in a transverse direction relative to the longest sides of the plate. The tube apertures 20 are formed by a punching operation and so define a peripheral flange 22 which projects upwardly from the upper surface 24 of the plate 10, as viewed in FIG. 1 (and as seen most clearly in FIG. 2).

Plate 10 is preformed to the configuration shown in FIG. 1, ready for use as an outlet plate 10 for a secondary heat exchanger assembly according to an exemplary embodiment of the invention. As can be seen, the edges 26 of the plate 10 are turned to project downwardly as viewed in FIG. 1, to form an open box type structure (as would be apparent if viewed from below in FIG. 1).

Referring now to FIG. 3 there is a shown a plate 30 of rectangular form, and including three fixing apertures 32 arranged at predetermined locations along one of the longer sides 34 of the plate 30.

Plate 30 also includes an array of tube apertures 36 arranged centrally on the plate 30 in a plurality of rows extending parallel with the longest sides 34 of the plate 10. Again, adjacent rows in the array of tube apertures 36 are staggered relative to one another. The tube apertures 36 are formed by a punching operation and so define a peripheral flange 38 which projects downwardly from the lower surface 40 of the plate 30, as viewed in FIG. 3 (and as seen most clearly in FIG. 4).

Plate 30 is pre-formed to the configuration shown in FIG. 3, ready for use as an inlet plate 30 for a secondary heat exchanger according to an exemplary embodiment of the invention.

Unlike the plate 10 in FIG. 1, plate 30 does not include turned edges. However, the position of the tube apertures 36 on the plate 30 matches the position of the tube apertures 20 on plate 10.

According to an exemplary method of assembly, the two plates 10 and 30 are arranged in a spaced array with a plurality of stainless steel heat exchanger tubes 50 arranged therebetween, e.g. as shown in FIG. 5. The plates 10 and 30 and tubes 50 are arranged with the tubes 50 in alignment with the tube apertures 20, 36, e.g. using a special purpose assembly machine. The plates 10 and 30 are then moved in the direction of one another, to force the ends of the tubes 50 into the respective tube apertures 20, 36. The diameter of the tube apertures 20, 36 is selected to create an interference fit between the associated flanges 22, 38 and the respective ends of the tubes 50. In an exemplary method of assembly, a swaging tool (not shown) may be used to swage the ends of the tubes 50 in the flanges 22, 38 to securely couple the tubes 50 to the inlet/outlet plates 12, 30. The two ends may be swaged simultaneously, to reduce process time.

An example of a heat exchanger tube 50 for use in the secondary heat exchanger assembly and method of assembly described above is shown in FIG. 6. The tube 50 is of seamless drawn construction, having a predefined outer diameter (for providing on an interference fit with the flanges 22, 38 on the plates 10, 30) and a predefined internal diameter. In exemplary embodiments, the tubes 50 are of small diameter.

Test results have shown a 6 mm diameter bore to provide beneficial performance characteristics. In exemplary embodiments, the diameter of the tubes is within the range 4 mm to 8 mm.

A spiral thread 52 is provided as a recessed formation in the outer surface of the tube 50. The configuration of the thread 52 (in terms of pitch and depth relative to the length of the tube) is configured to promote the flow of combustion gases though the tube 50, in use.

Test results show that a pitch of the spiral formation in the region of 6-12 mm (e.g.12 mm), at a depth of 0.69-0.7 mm, with a wall thickness of 0.5±0.03 mm provides optimum strength and heat transfer characteristics for exemplary embodiments.

In exemplary embodiments, the tube 50 is drawn and then the spiral thread 52 is formed on the drawn tube 50. A second drawing operation is then carried out to remove any significant deformations generated when the thread is formed, so as to maintain the accuracy of the tube diameter axial alignment, to promote optimum performance and avoid condensate being trapped within the tube.

The spiral thread 52 does not extend to the ends 54 of the tube 50; the thread 52 begins at a predetermined distance from one end 54 of the tube 50 and stops at a predetermined distance before the opposite end 54 of the tube 50, so that the ends 54 of the tube are plain, to ensure a tight fit with the tube apertures 20, 36 on the plates 10, 30.

A primary heat exchanger assembly 60 according to an exemplary embodiment of the invention will now be described with reference to FIG. 7.

The primary heat exchanger assembly 60 has a cylindrical drum 62 of pre-selected diameter. An outlet plate 64 having a central outlet aperture 66 is fitted to the lower end of the drum 62, as viewed in FIG. 7, and affixed thereto (e.g. by seam welding to a peripheral flange 68) to form an airtight seal with the drum 62.

An inlet assembly 70 is fitted to the upper end of the drum 62, with a thermal insulation plate 72 fixed in place beneath the inlet assembly 70. The inlet assembly 70 affixed to the drum (e.g. by seam welding to a peripheral flange 68) to form an airtight seal with the drum 62.

X-type strengthening formations 74 are formed adjacent each of the corners 76 of the outlet plate 64. These serve as strengthening braces and also allow the material to move during the expansion and contraction cycles. Allowing the material to move eliminates noise issues which would otherwise result, such as ‘bonging’ and ‘ticking’, due to the different expansion rates of the mating materials.

Referring now to FIG. 8, it can be seen that the primary and secondary heat exchanger assemblies 60, 40 form separate sub assemblies of a condensing heat exchanger according to an exemplary embodiment of the invention. The two sub assemblies 60, 40 are then brought together (e.g. using a mechanical jig) and attached to one another. In the illustrated embodiment, the two assemblies 60, 40 are affixed to one another by a seam weld 78 formed between respective edges regions of the outlet plate 64 of the primary heat exchanger assembly 60 and the inlet plate 30 of the secondary heat exchanger assembly 40.

As can be seen from FIG. 9, a box closure 80 is then be applied over the lower end of the united sub assemblies 40, 60. The box closure 80 includes a peripheral flange 82 and a seam weld is used to affix united plates 30, 64 to the peripheral flange 82, and create an airtight seal between the closure 80 and the united sub assemblies 40, 60.

FIGS. 10A and 10B show a box closure 80 of exemplary configuration, having side walls 84 and a base wall 86. An array of concentric circles 88 is pressed into the base wall 86 (on the outer side of the box 80). The base wall 80 is then pressed outwards, to extend beyond the side walls 84 (e.g. as viewed in FIG. 10B), and bent so as to define four generally triangular sloping surfaces 90. This configuration has been found to provide reduced noise from flexure of the material (e.g. bongs and ticks) and reduces the tendency for splitting, in use.

Once assembled, the finished assembly is ready for incorporation in a condensing heat exchanger, e.g. with a fuel burner (not shown) in communication with the inlet assembly 70 of the primary heat exchanger 60, so that combustion gasses pass through the drum 62, into the box closure 80 and out through the tubes 50 of the secondary heat exchanger 40.

The provision of an array of small diameter tubes as the secondary heat exchanger has been found to be particularly effective, especially when incorporating a spiral thread and/or when arranged in staggered rows. The arrangement of the tubes between the pre-formed inlet and outlet plates provides a convenient and efficient sub-assembly, which can be readily incorporated with the primary heat exchanger sub-assembly described herein. 

1. A condensing heat exchanger of the kind having a primary heat exchanger and a secondary heat exchanger connected downstream of and in series with the primary heat exchanger, in which the secondary heat exchanger is arranged to at least partially condense combustion gases discharged from the primary heat exchanger, wherein the primary heat exchanger comprises a drum having a longitudinal axis and the secondary heat exchanger comprises a plurality of tubes, wherein each tube has a longitudinal axis and an outer surface which includes a spiral formation concentric with the longitudinal axis of the tube, and wherein the tubes are arranged to extend parallel with the longitudinal axis of the drum.
 2. (canceled)
 3. A condensing heat exchanger according to claim 1 wherein the spiral formation begins at a predetermined distance from one end of the tube and stops at a predetermined distance from the opposite end of the tube, so that the ends of the tube are plain.
 4. A condensing heat exchanger according to claim 1 wherein the tubes have a diameter in the range of approximately 4 mm to 8 mm.
 5. (canceled)
 6. A condensing heat exchanger according to claim 1 wherein the tubes are provided between inlet and outlet plates, as a subassembly of the heat exchanger.
 7. A condensing heat exchanger according to claim 6 wherein the inlet plate and outlet plate are preformed with tube apertures for receiving the ends of the tubes in a pre-defined array.
 8. A condensing heat exchanger according to claim 7 wherein each tube aperture has a peripheral flange to provide a sleeve for receiving part of the length of a respective tube.
 9. A condensing heat exchanger according to claim 1 wherein the primary heat exchanger comprises a subassembly including a cylindrical drum sealingly attached to an outlet plate; wherein the secondary heat exchanger comprises a subassembly including said plurality of tubes sealing attached to an inlet plate; and wherein the two subassemblies are united with one another by attachment between the outlet plate and inlet plate.
 10. A condensing heat exchanger according to claim 9, further including a closure for connecting the two subassemblies and defining a passageway for combustion gases to be channelled from the primary heat exchanger to the secondary heat exchanger.
 11. A condensing heat exchanger according to claim 10 wherein the closure has a box-type configuration with four side walls and a base wall, wherein the base wall has a pressed configuration in which an outer surface of the base wall extends outwardly.
 12. A condensing heat exchanger according to claim 10, further including an array of concentric circles press-formed into a major surface of the closure.
 13. A method of assembling a condensing heat exchanger of the kind having a primary heat exchanger and a secondary heat exchanger connected downstream of and in series with the primary heat exchanger, and in which the secondary heat exchanger is arranged to at least partially condense combustion gases discharged from the primary heat exchanger, comprising the steps of: providing the primary heat exchanger in the form of a drum having a longitudinal axis, providing the secondary heat exchanger in the form of a plurality of tubes, and providing an inlet plate and an outlet plate for the tubes, the inlet and outlet plates being preformed with tube apertures for receiving the ends of the tubes in a pre-defined array, wherein the method further includes the steps of arranging the plates in a spaced array with said tubes arranged between the plates with the tubes in alignment with the tube apertures, and forcing the ends of the tubes into the respective tube apertures by driving the plates in the direction of one another, and then arranging the tubes to extend parallel with the longitudinal axis of the drum.
 14. (canceled)
 15. A method according to claim 14 wherein each tube aperture is provided with a peripheral flange projecting from the associated plate to provide a sleeve for part of the length of a respective tube.
 16. A method according to claim 15 wherein the peripheral flange is swaged to fixedly couple the flange to the tubes.
 17. A method according to claim 13 wherein a spiral thread is formed in the outer surface of each tube prior to incorporation in the assembly.
 18. A method according to claim 17 wherein each tube is drawn and then the spiral thread is formed on the drawn tube, prior to a second drawing operation, to remove any significant deformations generated when the thread is formed.
 19. A method according to claim 13 wherein: the primary heat exchanger is formed as a subassembly including a cylindrical drum sealingly attached to an outlet plate; the secondary heat exchanger is formed as a separate subassembly including said plurality of tubes sealing attached to an inlet plate; and the two subassemblies are united with one another by attachment between the outlet plate and inlet plate.
 20. A method according to claim 19 wherein a closure is attached to the united subassemblies to define a passageway for combustion gases to be channelled from the primary heat exchanger to the secondary heat exchanger.
 21. A method according to claim 20 wherein an array of concentric circles is pressed into a surface of the closure to reduce noise during flexure of the closure under thermal expansion.
 22. A method according to claim 20 wherein the closure is of pre-formed box-type configuration having four side walls and a base wall, wherein the base wall is pressed outwards, to reduce noise during flexure of the closure under thermal expansion. 